Verify global's return value instead of checking whether the output is empty

When calling helm-gtags-find-tag-from-here in a new file which is not indexed by gtags (this happens a lot when developing new code), global will fail with "global: path name in the context is not found.". But the code doesn't handle this kind of error, which leads an empty buffer "global" being created.

It's more reliable to verify global's return value.